The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-base superalloys and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc. Aluminum-containing coatings, particularly diffusion aluminide coatings, have found widespread use as environmental coatings on gas turbine engine components. Aluminide coatings are generally formed by a diffusion process such as pack cementation or vapor phase aluminizing (VPA) techniques, or by diffusing aluminum deposited by chemical vapor deposition (CVD) or slurry coating. During high temperature exposure in air, an aluminide coating forms a protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the coating and the underlying substrate.
Slurry coatings used to form aluminide coatings contain an aluminum powder in an inorganic binder, and are directly applied to the surface to be aluminized. Aluminizing occurs as a result of heating the component in a non-oxidizing atmosphere or vacuum to a temperature that is maintained for a duration sufficient to melt the aluminum powder and diffuse the molten aluminum into the surface. Slurry coatings may contain a carrier (activator), such as an alkali metal halide, which vaporizes and reacts with the aluminum powder to form a volatile aluminum halide, which then reacts at the component surface to form the aluminide coating.
During a typical diffusion coating method, either CVD or slurry coating, the furnace is typically in a dynamic state with respect to the atmosphere within the furnace. For example, in both slurry and gel coating diffusion heat treating methods, a treatment cycle is typically performed using a vacuum furnace. That is, there is typically a pumping system attached to the exhaust system of the furnace to remove gas from the furnace, to keep gas flowing and/or to maintain a reduced pressure within the furnace.
However, the necessary components associated with such a dynamic system (e.g., furnace walls, heat zone, pump lines, oil, booster and mechanical pumps, blower motor, etc.) are exposed to the deposition and reaction gases. Such exposure can result in activator deposits on the components within the dynamic system, which can significantly shorten their working life span and cause multiple manufacturing issues and delays. As such, a need exists for an improved diffusion coating method to form and repair aluminide coatings.